1. Field of the Invention
The present invention relates to an integrated optical circuit and, more specifically, to a method and a device for aligning an optical fiber and a waveguide formed at the surface of an integrated optical circuit.
2. Discussion of the Related Art
Integrated optical circuits are more and more used in telecommunications, especially for the transmission, the processing, or the storage of data. Integrated optical circuits may have many functions, such as multiplexing, demultiplexing, modulation, demodulation, spectral routing, amplification, storage, filtering, resonator functions, etc.
Integrated optical or optoelectronic circuits are generally formed in and on semiconductor wafers similar to those used in microelectronics. An integrated optical circuit comprises one or several elementary optical components processing one or several light beams, the light beams being conveyed between elementary optical components by optical waveguides.
The integration of an increasing number of functions on a same chip requires the miniaturization of integrated optical circuits and of the associated optical waveguides. When waveguides have dimensions below one micrometer, it is spoken of submicronic or nanometric waveguides. Currently, waveguides may have cross-sections on the order of 0.5×0.2 μm2.
For mid and long range transmissions, that is, within a range from a few meters to several kilometers, optical fibers are the preferred optical transportation means. An optical fiber usable in the visible and close infrared range currently has a diameter ranging between 10 μm and a few tens of micrometers. Accordingly, it is necessary to use light coupling systems between optical fibers and submicronic waveguides to compensate for the size mismatch imposed by such structures.
FIG. 1 is a perspective view illustrating a known structure for coupling an optical fiber and a submicronic waveguide associated with an integrated optical circuit. This anamorphotic structure is generally called “inverse taper” in the art, after its shape.
The structure of FIG. 1 is formed on a silicon substrate 1 covered with an insulating layer 3, for example made of silicon oxide. A wide waveguide 5, for example made of silicon oxide SiOx, having an optical index ranging between 1.6 and 1.8, is formed on insulating layer 3. Wide guide 5 typically has a cross-section with dimensions on the order of a few micrometers, for example, a 3-μm width and a 1-μm height, and is intended to be illuminated by an optical fiber (shown in FIG. 1 by an arrow 7) at a first one of its ends, substantially above an edge of support 1.
A submicronic optical waveguide 9, formed at the surface of layer 3, extends into wide waveguide 5 and progressively narrows therein to form a tip 11 on the side of the first end of wide waveguide 5. Submicronic waveguide 9 and tip 11 may be made of silicon (having an optical index of 3.47). It should be noted that an insulating layer, not shown, for example made of stoichiometric silicon oxide of optical index equal to 1.44, extends on top of optical waveguides 5 and 9 to confine the light beams in these waveguides.
In normal operation, a light beam of adapted wavelength and polarization penetrating into wide waveguide 5 enters submicronic waveguide 9. Conversely, a light beam conveyed by submicronic waveguide 9 penetrates into wide waveguide 5.
FIG. 2 illustrates a simplified example of optical inputs/outputs of a chip comprising an integrated optical circuit 13. Many wide waveguides 5 having their first ends located substantially above the chip edges extend on a silicon oxide layer 3 formed on a silicon support. Each wide waveguide 5 is coupled to a submicronic waveguide 9. Submicronic waveguides 9 are connected to integrated optical circuit 13, for example carrying out one or several of the above mentioned functions. As an example, integrated circuit chip 3 may have a surface area ranging between 1 mm2 and 4 cm2 and integrated optical circuit 13 may take up almost the entire surface area.
Wide waveguides 5 have cross-sections on the order of a few square millimeters (for example, with a side length between 1 and 4 μm). The coupling with an optical fiber typically having a diameter on the order of 10 μm is performed via an optical system comprising one or several lenses, another possibility being for the end of each optical fiber to be given a shape ensuring a lens effect.
For the circuit of FIG. 2 to operate properly, each optical fiber must be perfectly aligned with the wide waveguide associated thereto. Several methods have been provided to form this alignment. For example, the integrated optical circuit may be provided to deliver a light beam at the output of wide waveguide 5 and the optical fiber is considered as being aligned when the amount of light received by said fiber is maximum. It is also possible to illuminate wide waveguide 5 with the optical fiber and to detect a light intensity maximum in the submicronic circuit.
However, such methods pose several problems. First, they require providing, in the integrated optical circuit, elements dedicated to the alignment of the optical fibers, for example, light outputs or photodetectors. Further, in the alignment, the integrated optical circuit must be in operation, and thus, for example, electrically supplied. Finally, the wavelength of the light beams used for the alignment necessarily is that of the light beams used in the integrated optical circuit.
There is a need for a device and a method enabling to align an optical fiber on an optical waveguide, independently from the associated integrated optical circuit, from its operating mode, from its operating wavelengths, and from the light polarization states that it requires.
Patent application WO 2004/088801 provides a device comprising, at the surface of a support, a first optical waveguide coupled to a second waveguide of smaller size at one of its ends. A diffraction grating, formed at the surface of the first or second waveguides, is sized to filter beams exhibiting predetermined wavelengths.